Semiconductor devices typically require thin oxide layers to be formed at various stages of their fabrication. For example, in transistors, a thin gate oxide layer may be formed as part of a gate stack structure, including sidewalls, as will be described further below. In addition, in some applications, such as in the fabrication of a flash memory film stack, a thin oxide layer may be formed surrounding the entire gate stack, for example, via exposing the stack to an oxidation process. Such oxidation processes have conventionally been performed either thermally or using a plasma.
Thermal processes for forming oxide layers, for example, the gate oxide layer or the gate stack oxidation layer, have worked relatively well in fabrication of semiconductor devices of the larger feature sizes used in the past. Unfortunately, as feature sizes are becoming much smaller and different oxides are employed in the next generation of advanced technologies, the high wafer or substrate temperatures often required in thermal oxidation processes are problematic in that the dopants in the silicon wafer (well doped and junctions) diffuse at the higher temperatures (e.g., above about 700° C.). Such a distortion of the dopant profiles and other features can lead to poor device performance or failure.
The use of lower temperature or “cold” processing methods is increasing in microelectronic manufacturing due to the minimization of dopant redistribution and high-temperature related defects. Despite these benefits, plasma processes used to form oxide layers have problems to those presented by thermal oxidation processing. For example, plasma oxides can contain fatal defects in the gate oxide structure such as dangling bonds. For example, conventional oxidation processes often result in a defect known as a bird's beak. Bird's beak refers to diffusion of the oxide layer into the layers of the film stack structure from the sides at the interface between adjacent layers, rounding off the corners of the adjacent layers. The resultant oxide shape has a profile that resembles a bird's beak. The intrusion of the oxide layer into the active region of the memory cell (e.g., in flash memory applications) reduces the active width of the memory cell, thereby undesirably reducing the effective width of the cell and degrading the performance of the flash memory device.
Utilizing a plasma oxidation process to form oxide layers on substrate, for example silicon substrate, forms a plasma in the reactor which consumes the silicon substrate to forms a SiO2 oxide layer on the silicon substrate. For at least this reason, plasma oxidation is thought to more likely produce an ideal Si—SiO2 interface, which leads to high device performance and reliability. Further, plasma oxidation grows SiO2 oxide layers having high electrical quality rapidly and at lower temperatures than thermal oxidation processes. Despite the lower processing temperature, the quality of the resulting oxide layers can degrade with reducing process temperature. This degradation of quality becomes more pronounced with oxide layers formed by plasma oxidation, when the oxidation temperature is below 700° C. Thus, there is a need for an improved method for forming oxide layers at lower temperatures.